It has now been found that the in vivo viscosity of blood varies among individuals, and that an in vivo viscosity measurement of blood substantially corresponding to the viscosity when in the body can be of value to the physicians in the diagnosis of certain body conditions. The present invention provides means for rapidly and inexpensively determining comparative substantially in vivo blood viscosity. The measurement of blood viscosity long after removal from the patient has been carried out in the past. However, viscosity of whole blood has not been studied thoroughly because of the difficulties in the settling of erythrocytes and of the very dramatic changes that occur in blood following its withdrawal from the cardiovascular system.